Tried OTA update, didn't work, update failed.
Removed Team Win Recovery (part of Safestrap), tried OTA update. Still update failed.
Went to DroidLife, found that if I had hidden or deleted any bloatware included by Verizon the OTA will fail. Fortunately I merely froze them instead of deleted them (I think) with Titanium Backup. So I defrosted them.
Trying OTA download... Again.
Before proceeding, I "saved root" by downloading and running "OTA Rootkeeper" by Voodoo.
This should allow me to keep root if the JB OTA update installs properly. Though there may be some manual steps involved, according to XDA developers.
If JB OTA fails again, I had the JB update image downloaded and I'll try to install it using recovery.
Just in case, as a backup, I'm going to reinstall Android SDK/ADT to get Android Debug Bridge working. I have the sync cable here somewhere... :)
When I got the update working, with JB, I'll post the step by step guide.